# wgrib2: -if_n

## Introduction

The -if_n option is an -if for a range of inventory numbers.
The -if_n option uses the same syntax as the "for_n" option.
If you want to operate on
inventory records 10 to 20, you would use the parameter 10:20.
If you want to operate on all the even inventory records from 10 to 20, you would
use 10:20:2. The restrictions are the start value must be less than
the ending value and the step has to be greater than zero.

The -if_n option is similar to the
-for_n option in that they both select a range of invetory records.
The difference is that the -for_n option selects the range
of records that wgrib2 will process. With the
the -if_n option, all the records will be processed
but only the additional processing within the -if block will be only
done for the selected records. The -for_n option can be much
faster if the field has to be decoded. With the -for_n option,
only the selected records need to be docoded whereas all the records would be decoded
when using the -if_n option. The command line, however,
can have multiple -if_n options.

## Usage

```
-if_n I:J:K        same as for n = I to J by K
-if_n I:J          same as for n = I to J by 1
-if_n I::K         same as for n = I to MAX_INTEGER by K
-if_n I            same as for n = I to MAX_INTEGER by 1
```

### Example

```
 $ wgrib2 file.grb2 -if\_n 4:5 -s -fi
4:13335:d=2008120200:RH:750 mb:anl:
5:17098:d=2008120200:TMP:2743 m above mean sea level:anl:
```

See also: [-match](./match.md)
[-if_rec](./if_rec.md)
[-for](./for.md)
[-for_n](./for_n.md)

---

> Description: if X if (inv numbers in range), X=(start:end:step)

_Docs derived from <https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/if_n.html>_
